Along with the technical development on mobile instruments as well as the increased demands thereon, more secondary batteries are needed as an energy source. Among such secondary batteries, lithium secondary batteries are widely used due to high energy density and voltage, long life cycle and low self-discharge rate.
Lithium-containing cobalt oxide (LiCoO2) is frequently used for a cathode of a lithium secondary battery as a cathode active material. Aside from it, lithium-containing manganese oxides such as LiMnO2 with a layered crystalline structure and LiMn2O4 with a spinel crystalline structure and lithium nickel based oxides such as LiNiO2 comprising lithium and nickel together are also considered.
Among the above cathode active materials, LiCoO2 is currently widely used due to its excellent properties, for example excellent cycle characteristics. However, LiCoO2 has bad safety and is very expensive since its raw material, cobalt, is very rare. In addition, LiCoO2 is not suitable for mass production, which is a prerequisite as a power source for electric vehicles or the like.
Lithium manganese oxides such as LiMnO2 and LiMn2O4 are advantageous in that their raw material, manganese, is abundant and nature-friendly. In this reason, lithium manganese oxides attract much interest as a cathode active material substituting for LiCoO2. However, such lithium manganese oxides have small capacity and bad cycle characteristics.
Meanwhile, lithium nickel based oxides such as LiNiO2 are cheaper than the cobalt based oxides and nevertheless exhibit high discharge capacity when being charged to 4.3V. In addition, a doped LiNiO2 has a reversible capacity approaching about 200 mAh/g, which exceeds a capacity of LiCoO2 (about 165 mAh/g). Thus, a battery using the lithium nickel based oxide as a cathode has an improved energy density, so the lithium nickel based oxide is actively studied to develop high-capacity batteries.
However, secondary batteries adopting lithium nickel based oxides are not easily put into practice due to the following reasons.
First, charge/discharge cycle characteristics are not good.
Second, nickel has greater reactivity than cobalt and manganese. Thus, when a battery is stored at a high temperature or charged/discharged in a fully-charged state, the swelling phenomenon caused by a decomposition reaction of nonaqueous electrolyte becomes more serious. As a result, particularly in case of angled batteries and pouch batteries, the thickness of battery is increased, and it is very disadvantageous in a set such as cellular phones and notebooks. In other words, when being left alone at a high temperature, such batteries are not safe.